Many optical assemblies, particularly those used for such purposes as lithographic projection or semi-conductor inspection, have strict requirements for optical performance such as high transmission, low pupil non-uniformity, low RMS wavefront errors, and low transmitted wavefront asymmetries. Design and tolerance considerations play important roles in meeting wavefront performance requirements. For example, design forms are chosen to reduce sensitivity to expected types of errors accompanying the manufacture of optical components, and tolerances of the optical components are chosen with respect to the design sensitivities so that manufacturing variations do not severely influence the system level performance. Nevertheless, the assembly or “build” of even properly designed and manufactured components can introduce additional wavefront errors that degrade overall performance For example, assembly techniques such as bolting, threading, or otherwise fastening the optical components together can produce mechanical stresses or strains on the optical components that can degrade wavefront performance of an optical assembly.
A variety of methods have been used to isolate optical components from stresses transferred through their mountings. For example, flexures or semi-kinematic style mounting methods have been incorporated into lens holders (e.g., “cells”). These methods isolate the lenses from the stresses or strains produced by fastening adjacent lens holders or spacers together to form an integral lens assembly. Other methods use highly compliant adhesive layers between the lens elements and the holders to reduce deformations of the optical elements within the lens assemblies.
Methods have also been used to adjust optical elements based on performance measurements. For example, lenses or groups of lenses have been rotated or otherwise adjusted relative to one another to optimize measured performance. Yet another method uses a “correction element” that is deterministically manufactured to cancel measured wavefront errors of otherwise completed lens assemblies.